[Female mate choice, male-male competition, and male sexual traits: Experimental study of sexual selection directedness in Campbell's dwarf hamster (Phodopus campbelli Thomas 1905)]

We studied the relationship between female mate choice and male-male competition in Campbell's dwarf hamster. Sexually motivated (SM) females chose between two tethered male siblings that differed in expression of sexual traits. Males were unrelated to the female and able to contact and copulate with her. The same males were used in encounter experiment with the following free access of competing male to another SM female. We measured males' sex-related morphology of body mass, mid-ventral specific skin gland, ano-genital distance, and external testicular diameter. We also estimated levels of blood testosterone and cortisol, specific T- and B-cell immune responses to antigens, as well as aggressive and sexual dominance. We found out that female mate choice was not dependent on male aggressive and sexual dominance estimated through encounter experiments. Both in female mate choice and male-male competition experiments (encounter of males and free excess of males to receptive female) successful males did not differ from unsuccessful males in expression of sexual traits (ST). Among males with higher expression of ST mate choice by the female was not associated with aggressive dominance and sexual dominance of a male. There were no significant differences between successful and unsuccessful males in variables treated as potential predictors of male's success, both in female mate choice and male-male competition experiments. Variables explaining female's preference and male's aggressive dominance, which were revealed by General Regression Model analysis, did not coincide. Our results suggest that there are no reasons to discuss the expression of ST in Campbell dwarf hamster males in terms of intersexual or intrasexual selection mechanisms or in terms of their interaction. At the same time, our study demonstrates that testing theoretical models such as "Handicap hypothesis" or "Immunity handicap hypothesis" may face difficulties, which are discussed in the text.